Steve, depending on what kind of Pantera owner you are, PPC-Reno sells a polycarbonate plastic rear window that fits either size rear opening, is lighter and easier to handle and is cheaper to buy and to ship. Not stock, obviously.
Busted rear windows usually happen when a shop (or an owner) tries doing an engine change without first looking at the FAQs that say 'pull the distributor cap and the Weber carbs (if installed) before lifting & tilting the engine, or broken glass will result'.
Please note-I don't work for any vendor but I do keep up with whats available from them, for all our benefits.
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